Strain-leaf burr height detector

ABSTRACT

A burr height detector includes a thin flexible strain leaf with an active strain gage located close to an end of the strain leaf. The strain leaf is pressed into engagement with a burr and adjacent workpiece surface and flexes to apply variable strain to the active strain gage depending on burr height. The change in strain gage resistance is measured by a resistance bridge. An array of burr height detectors on a pressure head can be provided.

0 United States Patent 1 [111 3,777,558 Bain 1 Dec. 11, 1973 [54]STRAIN-LEAF BURR HEIGHT DETECTOR 3,116,469 12/1963 Wu 73/885 R UX [75]Inventor: James Primary Examiner-Richard C. Queisser [73] Assignee:General Electric Company, Assistant Examiner-John P. BeauchampSchenectady, N.Y. Attorney-John F. Ahern et al.

[22] Filed: Nov. 15, 1972 I [57] ABSTRACT [211 App 306599 A burr heightdetector includes a thin flexible strain leaf with an active strain gagelocated close to an end [52] U.S. Cl. 73/105, 33/172 E of the strainleaf. The strain leaf is pressed into en- [51] Int. Cl. 601]) 5/28gagement with a burr and adjacent workpiece surface [58] Field of Search73/105, 104, 88.5 R; and flexes to apply variable strain to the activestrain 33/DIG. 13, 172 E gage depending on burr height. The change instrain gage resistance ismeasured by a resistance bridge. An [56]References Cited array of burr height detectors on a pressure head canUNITED STATES PATENTS be provlded- 2,487,681 1l/l949 Weisselberg 73/885R UX 10 Claims, 6 Drawing Figures a l4 l0 l5 '3 tee.

PAIENIEMEBH m E 3377558 sum 20F 2 ACTIVE GAGE TEMPERATURE COMPENSATINGGAGE E 5 i {27 we r r25 26 1 STRAIN-LEAF BURR HEIGHT DETECTORBACKGROUND.- OF THE INVENTION This invention relates to a strain-leafgaging device for detecting and measuring. the height of burrsonworkpieces, as for example the burrs on sheet metal laminations usedinthe. manufacture of generators and other electric machines.

Several different techniques that may be: suggested for detecting burrsare not fully. satisfactory as well as not being convenient and readilyautomated. Burr detection can be achieved using low angle optical.illumination and reflection, by employing an. array of verticallymovable feelers similar to phonograph needles, and by making impressionsin thin insulating sheets with'attendant resistance tests. The height ofthe burr is not easily measured by any of these methods. A manualmicrometer measurement can be made, of course, but this frequentlycrushes the burr and the results in general are not reproducible andreliable because of the subjective evaluation made by the operator asto. the engaging pressure. Although the determination of burr height hasmany applications, it is particularly useful in the quality control ofstamped sheet metal laminations for electric machines such asgenerators, to determine when deburring is necessary or when to changethe punch press die. These laminations are stacked upon one another withinsulation between, and excessively long burrs defeat the function ofthe insulation by making unwanted electrical contact with an adjacentmetal lamination.

SUMMARY OF THE INVENTION In accordance with the invention, burr heightdetection and gaging is performed by means of a very thin, flexiblestrain leaf with an attached active strain gage located in an area thatflexes as a function of burr height when that portion of the strain leafis pressed into engagement with the burr and adjacent workpiece surface.The change of resistance of the active strain gage, which varies withburr height, is detected by a suitable resistance measuring circuit suchas a resistance bridge. a

In the preferred embodiment, the active strain gage is located veryclose to the end of the strain leaf that engages the burr, which end isnarrowed so that the active strain gage is subjected predominantly tobending forces and little or no twisting forces. A temperaturecompensating gage is attached to a substantially un strained portion ofthe strain leaf. The strain leaf is typically made of a hard metal andthe gages are bonded strain gages. Evenly distributed pressure isapplied to at least the end of the strain leaf that flexes, for exampleby using a yieldable member and overlyingpressure distributing membersecured to that end of the strain leaf. To simultaneously gage theburred edges of a sheet metal generator lamination or other workpiece,

an array of burr height detectors is arranged on a pressure head forjoint movement toward and away from the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of theflexible strain leaf per se showing an active strain gage on thenarrowed gaging tip and a temperature compensating strain gage;

FIG. 2.is a diagrammatic side view. illustrating the flexible strainleaf, in operative position sensing, the, height of aburr onasheetmetallamination;

FIG. 3 isadiagrammatic side view of the workpiece and flexiblestrainleaf with one form of suitable struC. ture for applying evenlydistributedpressure to thegaging tip;

FIG; 4. is a schematic circuit diagram of the equivalent resistances.for the active and temperature cornpensating strain. gages. connectedin, a Wheatstone bridge circuit for measuring the burr height;

FIG. 5' isa plan view of a stamped: sheet metal, genera.- torlamination; and

FIG. 6 is. a diagrammatic elevational view of a press sure head with anarray of strain-leaf gages for measur- DESCRIPTION OF THE PREFERREDEMBODIMENT The detection and. measurement of the height of burrs onworkpieces is performed by a gaging device incorporating a strainsensing element 10 of the type shown in FIG. 1. Strain sensing-element10 is comprised by a very thin flexible strain leaf 1 1 to which isattached at least one active strain gage 12 and, preferably, a passiveor temperature compensating strain gage 13. Strain leaf 11 is typicallyseveral thousandths of an inch in thickness and is made of a hard metalsuch as hard steel, stainless steel, or even tungsten carbide in orderto be resistant to wear by being repeatedly pressed into engagement withthe burrededge of a workpiece. Active strain gage 12 is a miniaturizedgage and is mounted very close to one end of strain leaf I1 so thatsmall burrs, in the order of 0.006 inch or less in height, producemeasurable strains. Rather than using a rectangular strain leaf, bestresults are obtained by attaching active strain gage 12 to a centrallylocated, reduced width gaging tip 11a at one end of strain leaf 11. Withthis construction, active strain gage 12 is re sponsive primarily tobending forces when placed in operation as shown in FIG. 2, and is notsubjected to error by reason of also responding to twisting forces.

Strain gages I2 and 13 are miniaturized, bonded, electrical resistancestrain gages. Active strain gage 12 comprises a relatively thin metallicconductor formed inaccordian fashion with a pair of contact pads 12a and12b on an insulating substrate 14. Conventional strain gages that areavailable commercially can be used in the practice of the invention,such as one type in which the strain gage and contact pads arefabricated by printed circuit techniques on a plastic substrate.Temperature compensating strain gage 13 is preferably identical to theactive strain gage, and the unit further includes a pair of contact pads13a and 13b formed with the strain gage on substrate 15. Conductors 16,which can be discrete wires, connect the strain gages in a suitableresistance measuring circuit such asthe half bridge circuit shown inFIG. 4. As has been mentioned, temperature compensating strain gage 13is a nonstrain responsive component in the resistance measuring circuitand thus is mounted toward the rear of strain leaf 11 where it remainsrelatively flat and is not subjected to bending or twisting forces. Toillustrate typical dimensions, strain leaf 11 has an overall length of 1inch, an over-all width of k inch, and gaging tip 11a has a width of /2inch and a parallellength of threesixteenths inch. The thickness of theleaf is 0.004 inch.

FIG. 2 illustrates the principle of operation of strainleaf sensingelement when used to measure the height of burrs 17 at one edge of aworkpiece 18. Workpiece 18, by way of illustration, is a sheet metallamination punched out by a punch press that produces the burrs 17 asthe result of the shearing action. The gaging tip of strain leaf 11 isaligned with the burred edge of workpiece 18, such that the end of thegaging tip engages burr 17 and projects only a slight distance beyondwhen a downward pressure P presses strain leaf 11 into engagement withthe workpiece. The pressure P causes the gaging tip 11a to flex andapply strain to active strain gage 12a, the amount of applied strainbeing a function of the height of the burr 17 being measured. Beyond thepoint of tangency 19, strain leaf 11 is not subjected to bending forcessince it is pressed flat against the upper surface of workpiece 18. Whengaging a burr 17 that is 0.0003 inch in height, for instance, the pointof tangency 19 is 0.060 inch from the edge of the workpiece. The pointof tangency remains approximately fixed as the height of the burr beinggaged varies. At this scale temperature compensating gage 13 ordinarilywould not appear, but is illustrated to show that it is located in anarea not subjected to strain.

The applied pressure P is preferably evenly distributed over the fulllength of the parallel portion of the gaging tip 11a. For the specificstrain-leaf sensing element being described, an evenly distributedpressure of 60 lbs/sq. in. gives good results. While it is not incorrectto apply the pressure over the full length of strain leaf 11, this isnot required. A preferred pressure applying structure is illustrated inFIG. 3. In this diagram strain leaf 11 is mounted in cantilever fashionwith the end remote from gaging tip 11a fixedly mounted in appropriatemanner, although the entire strain leaf 11 can also be mounted to moveup and down as a unit. A yieldable pressure applying member 20 made ofsoft rubber or other material is secured, as by cementing, to the upperface of gaging tip 11a, extending over the full areaof the gaging tipand covering the active strain gage 12. A metallic disk 21 may besecured to the upper surface of yieldable pressure-applying member 20 todistribute the applied pressure. The application of pressure P to disk21 manually or by some suitable mechanical structure causes the strainleaf gaging tip 11a to be pressed into engagement with the burr 17 andthe upper surface of workpiece 18 as shown in FIG. 2. The flexed end ofgaging tip 11a is pressed upward into soft rubber member 20, whichyields. The remaining portions of strain leaf 11 not attached topressureapplying member 20 are, of course, not in engagement with theworkpiece 18, but this is not needed to obtain an accurate measurementof burr height.

In FIG. 4 is shown a resistance measuring bridge circuit for determiningthe height of a gaged burr. The equivalent resistances for active gage12 and temperature compensating gage 13 are connected in a Wheatstonebridge. In this arrangement, series-connected gages 12 and 13 areconnected across the terminals of a battery 22, and a balancingpotentiometer 23 is also connected to the battery. A null detector 24 isfurther connected to the junction of gages 12 and 13 and to the pointerof potentiometer 23. Since the resistance of active gage 12 changes as afunction of the applied strain, which varies with the height of the burrbeing gaged, the equivalent circuit representation is a variableresistor. Temperature compensating gage 13 is not subjected to strainand therefore is represented as a fixed resistor. Assuming propercalibration, the pointer of potentiometer 23 is adjusted until nulldetector 24 indicates a null, and the relative values of the twoportions of potentiometer 23 are indicative of the burr height. Inappropriate cases other resistance measuring circuits can be usedincluding commercially available amplifiers made specifically for thepurpose of reading strain gage resistance changes.

Although the strain-leaf burr height detector and gage here describedhas many applications, it is particularly useful to measure the heightof burrs on sheet metal laminations used to manufacture generators andother electric machines. A generator lamination 25, in one of the manyconfigurations in which they are manufactured, is illustrated in FIG. 5.These sheet metal laminations, made of electrical grade steel, arecommonly coated with shellac or other insulating material and stackedone upon the other to form a generator rotor or stator part. Theshearing action of the punch press on which they are fabricated leavesan upstanding burr at the edges of the lamination. Burr height must becontrolled thoroughly in generators, since if they get too large,adjacent laminations can short circuit and cause a generatormalfunction. The strain-leaf burr height detector functions either todetermine when a separate deburring step is needed, or when a change ofpunch press die is required. An array of strain-leaf burr heightdetectors can be used, all operating simultaneously and located atselected positions on the workpiece where it is known that burrs areformed or likely to be formed.

A preferred embodiment of the invention in which an array of strain-leafburr height detectors are mounted on a movable pressure head isillustrated in FIG. 6. A jig 26 is provided and has a number of stops 27positioned so as to accurately locate the sheet metal lamination 25. Apressure head assembly 28 has on its lower surface a plurality ofstrain-leaf burr height detector units as previously described, heredesignated by the numeral 29. The gaging tip of each individual strainleaf in each unit 29 is, of course, precisely located with regard to theburred edge of the workpiece to be gaged. The pressure head assembly 28is moved up and down into and out of engagement with the sheet metallamination 25, measuring the height of the burrs at each location. Themeasured output data can be fed to a computer to keep a continuous checkof the burr heights, can be fed to an alarm that signals an excessiveburr height, or used in some ,other manner. An advantageousconstruction, not here illustrated, for an array of strain-leaf burrdetectors is to use a single thin sheet of metal with localized areas ofplastic insulating material to which the individual active strain gagesare bonded. The thin sheet of metal with an array of strain gages is inturn bonded to a single thin sheet of rubber material.

. Another application is that a single strain-leaf burr height detectorcan be incoporated in a manual tool with a plier-like handle arrangementfor applying pressure during the gaging operation.

In summary, a burr height detector for general usage with a variety ofworkpieces is based on a unique application of the strain gageprinciple. By employing a thin flexible strain leaf and miniaturizedstrain gage, the strain applied to the strain gage is a function of burrheight when the strain leaf is pressed into engagement with the burr andadjacent workpiece surface. The strain-leaf burr height detector isconvenient, accurate, gives reproducible results and is capable ofautomation.

While the invention has been particularly shown and described withreference to several preferred embodiments, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What 1 claim as new and desire to secure'by Letters Patent of the UnitedStates is:

1. A burr height detector comprising a strain sensing element includinga flexible strain leaf and at least one attached active strain gage,

means for pressing at least a portion of said strain leaf intoengagement with a burr and adjacent workpiece surface to apply strain tosaid active strain gage that varies with the burr height, and

a circuit for detecting the change in resistance of said active straingage.

2. A construction according to claim 1 wherein said active strain gageis located relatively close to one end of said flexible strain leaf.

3. A construction according to claim 1 wherein said flexible strain leafhas a narrowed gaging tip, and

said active strain gage is located on said gaging tip relatively closeto one end of said strain leaf.

4. A construction according to claim 1 wherein said strain sensingelement further includes a temperature compensating strain gage attachedto a substantially unstrained portionof said strain leaf.

5. A construction according to claim 1 wherein said means for pressingat least a portion of said strain leaf into engagement with a burr andadjacent workpiece surface comprises a yieldable member and overlyingpressure distributing member attached to said portion of said strainleaf.

6. A burr height detector comprising a strain sensing element includinga flexible strain leaf and an active strain gage attached relativelyclose to one end of said strain leaf,

pressure applying means forpressing said one end of said strain leafinto engagement with a burr and adjacent workpiece surface to therebyflex at least a portion of said strain leaf and apply variable strain tosaid active strain gage as a function of burr height, and

a resistance measuring circuit for measuring the change in resistance ofsaid active strain gage.

7. A construction according to claim 6 wherein said one end of saidstrain leaf to which said active strain gage is attached is relativelynarrow whereby said active strain gage is subjected onlyto bendingforces.

8. A construction according to claim 6 wherein said strain sensingelement further includes a temperature compensating strain gage attachedto a substantially unstrained portion of said strain leaf.

9. A construction according to claim 6 wherein said pressure applyingmeans comprises a yieldable member and overlying pressure distributingmember secured to t at least the flexed portion of said strain leaf.

workpiece.

1. A burr height detector comprising a strain sensing element includinga flexible strain leaf and at least one attached active strain gage,means for pressing at least a portion of said strain leaf intoengagement with a burr and adjacent workpiece surface to apply strain tosaid active strain gage that varies with the burr height, and a circuitfor detecting the change in resistance of said active strain gage.
 2. Aconstruction according to claim 1 wherein said active strain gage islocated relatively close to one end of said flexible strain leaf.
 3. Aconstruction according to claim 1 wherein said flexible strain leaf hasa narrowed gaging tip, and said active Strain gage is located on saidgaging tip relatively close to one end of said strain leaf.
 4. Aconstruction according to claim 1 wherein said strain sensing elementfurther includes a temperature compensating strain gage attached to asubstantially unstrained portion of said strain leaf.
 5. A constructionaccording to claim 1 wherein said means for pressing at least a portionof said strain leaf into engagement with a burr and adjacent workpiecesurface comprises a yieldable member and overlying pressure distributingmember attached to said portion of said strain leaf.
 6. A burr heightdetector comprising a strain sensing element including a flexible strainleaf and an active strain gage attached relatively close to one end ofsaid strain leaf, pressure applying means for pressing said one end ofsaid strain leaf into engagement with a burr and adjacent workpiecesurface to thereby flex at least a portion of said strain leaf and applyvariable strain to said active strain gage as a function of burr height,and a resistance measuring circuit for measuring the change inresistance of said active strain gage.
 7. A construction according toclaim 6 wherein said one end of said strain leaf to which said activestrain gage is attached is relatively narrow whereby said active straingage is subjected only to bending forces.
 8. A construction according toclaim 6 wherein said strain sensing element further includes atemperature compensating strain gage attached to a substantiallyunstrained portion of said strain leaf.
 9. A construction according toclaim 6 wherein said pressure applying means comprises a yieldablemember and overlying pressure distributing member secured to at leastthe flexed portion of said strain leaf.
 10. A construction including anarray of burr height detectors as defined in claim 6 arranged on apressure head for joint movement toward and away from the workpiece.